1. Field of the Invention
The present invention relates generally to a heat generation type flow sensor destined for applications where measurement of a flow rate (also referred to simply as flow) of a fluid medium such as intake air in an internal combustion engine of a motor vehicle is required for performing, for example, an air/fuel ratio control for the internal combustion engine. More particularly, the present invention is concerned with a heat generation type flow sensor which can ensure an enhanced detection sensitivity and a high reliability.
2. Description of Related Art
For better understanding of the concept underlying the present invention, description will first be made of conventional heat generation type flow sensors known heretofore by reference to the drawings. FIG. 12 shows in a top plan view a flow measuring element employed in a conventional heat generation type flow sensor, as described, for example, in Japanese PCT Application Publication No. 500490/1998, and FIG. 13 is a circuit diagram showing an equivalent circuit of a flow-rate measuring bridge circuit in which the heat generation type flow sensor is employed.
Referring to FIG. 12, the measuring element is comprised of a substrate 120, and a diaphragm 110 formed on the substrate 120. Provided on the diaphragm 110 are a heat generating resistor 112, a pair of first and second temperature detecting resistors 113 and 114, another pair of third and fourth temperature detecting resistors 115 and 116, and a connecting resistor 117 for interconnecting the third and fourth temperature detecting resistors 115 and 116. The diaphragm 110 is heated to a predetermined temperature by means of the heat generating resistor 112. Assuming that a fluid medium such as the air flows in the direction indicated by an arrow in FIG. 12, the diaphragm 110 is subjected to cooling by the flow of the fluid medium. In this conjunction, it is noted that the temperature detecting resistors 113 and 115 located upstream of the heat generating resistor 112 are cooled more intensively than the temperature detecting resistors 114 and 116 disposed downstream of the heat generating resistor 112. Thus, by detecting the difference in temperature between the upstream and downstream temperature detecting resistors, the flow rate of the fluid medium can be measured.
Next, referring to FIG. 13, description will be directed to the basic operation of the flow-rate measuring circuit in which the conventional heat generation type flow sensor is employed. As can be seen in FIG. 13, the first and second temperature detecting resistors 113 and 114 cooperate to form a first measuring bridge arm having an intermediate tap 133. On the other hand, the third and fourth temperature detecting resistors 115 and 116 and the connecting resistor 117 cooperate to form a second measuring bridge arm having two taps 134 and 135. The taps 134 and 135 are connected in series by means of adjusting resistors 145 and 146, wherein the serial circuitry is connected in parallel to the connecting resistor 117 with a tap 147 being led out from a junction between the adjusting resistors 145 and 146.
A tap 131 led out from a junction between the first temperature detecting resistor 113 and the fourth temperature detecting resistor 116 is connected to a power source (voltage source) while a tap 132 led out from a junction between the second temperature detecting resistor 114 and the third temperature detecting resistor 115 is connected to the ground. Parenthetically, the taps 131, 132, 133, 134 and 135 correspond to bonding pads of the measuring element shown in a top plan view of FIG. 14, as described later on. By adjusting resistance values of the adjusting resistors 145 and 146, the zero point of the flow-rate measuring bridge circuit can be adjusted.
In the flow sensor of the so-called temperature difference detection type structured as described above, temperature lowering at the upstream side of the heat generating resistor 112 is significant when the flow rate of the fluid medium is in a low range, presenting thus a high flow sensitivity. However, as the flow rate of the fluid medium increases, the temperature difference between the upstream side and the downstream side of the heat generating resistor 112 decreases with the flow sensitivity being correspondingly lowered. Ordinarily, no remarkable dependency is observed in the relations between the flow sensitivity on one hand and the sizes of the heat generating resistor and the diaphragm on the other hand. In general, the heat generation type flow sensor is practically so designed that the width of the strip-like heat generating resistor does not exceed one third (⅓) of the width of the diaphragm with a view to reducing the power consumption.
Furthermore, since such feedback control is ordinarily adopted that the temperature of the heat generating resistor 112 remains constant regardless of variation of the flow rate of the fluid medium, the temperature detecting resistors 113, 114, 115 and 116 tend to incur error in the detected flow rate due to a thermal lag in the response to the change or variation of the flow rate even though high responsivity of the heating current can be assured.
FIG. 14 shows in a top plan view a fluid flow measuring element 201 employed in another conventional heat generation type flow sensor described in Japanese Patent Application Laid-Open Publication No. 311750/1998 (JP-A-H10-311750). Referring to FIG. 14, the measuring element 201 is comprised of a substrate 220 and a diaphragm 210 formed on the substrate 220. Formed on the diaphragm 210 are heating conductors 202a and 202b and a temperature detecting resistor 204. Additionally, a fluid temperature detecting resistor 207 is deposited on the substrate 220. These resistance elements are connected to an external circuit 214 (see FIG. 15) by way of bonding pads 330a, 330b, 330c, 330d, 330e, 330f and 330g. 
As is shown in FIG. 15, the measuring element 201 includes a supporting member 213b on which the fluid temperature detecting resistor 207 is fixedly supported so that both surfaces of the fluid temperature detecting resistor 207 are exposed directly to the air flow. Further, mounted fixedly on the supporting member 213b is the external circuit 214 which is electrically connected to the measuring element 201 by means of bonding wires 216. Besides, the external circuit 214 and the wire-bonded portion (i.e., interconnected portion of the bonding wires 216, the measuring element 201 and the external circuit 214) are covered hermetically by a cap member 213a for the purpose of protection of the wire-bonded portion.
Turning back to FIG. 14, the heating current is fed to the heat generating resistors 202a and 202b so as to keep the temperature of the temperature detecting resistor 204 higher than that of the fluid temperature detecting resistor 207 by a predetermined temperature. Thus, the flow rate of the fluid medium such as the air or the like can be detected on the basis of the heating current flowing through the heat generating resistors 202a and 202b. The heat generating resistors 202a and 202b are connected in series to each other so that the same heating current flows through both the heat generating resistors 202a and 202b. Accordingly, by comparing difference in voltage between the upstream heat generating resistor 202a and the downstream heat generating resistor 202b, the direction of the fluid or air flow can be determined.
The flow sensor of heating current detection type structured as described above can certainly exhibit an enhanced responsivity to the change of the flow rate. However, this type sensor suffers a problem that the sensitivity is low in a low range of flow rate because of nonnegligible heat losses due to heat conduction to the substrate 220 and a cavity 211 by way of the diaphragm 210 when compared with the heat loss due to the heat transfer to the fluid medium flow from the heat generating resistors 202a and 202b. 
Furthermore, the measuring element 201 shown in FIG. 14 is implemented in such structure that the heat generating resistors 202a and 202b are formed on the diaphragm 210 with the temperature detecting resistor 204 being disposed between these heat generating resistors 202a and 202b, wherein no consideration is paid to the relation in size between the heat generating resistors 202a and 202b and the diaphragm. As a result of this, the loss due to heat transfer to the flow of the fluid medium remains low relative to the amount of heat generated by the heat generating resistors 202a and 202b, as a result of which the flow sensitivity is lowered, giving rise to a problem.
As is apparent from the foregoing, the heat generation type flow sensors in which the diaphragm is formed by removing partially the material of the substrate and in which the heat transfer from the heat generating resistors deposited on the diaphragm to the flow of the fluid medium such as air flow is made use of can be classified into two groups, i.e., the sensor of the temperature difference detection type designed for detecting the flow rate on the basis of the difference in temperature between the temperature detecting resistors disposed upstream and downstream of the heat generating resistor (FIGS. 12, 13) and the sensor of the heating current detection type which is designed for detecting the flow rate on the basis of the heating current flowing through the heat generating resistors (FIGS. 14, 15).
Of the flow sensors mentioned above, the temperature difference detection type flow sensor suffers a problem that temperature difference between the upstream and downstream regions becomes small in the range of high rate, which incurs lowering of the sensitivity although this type flow sensor can ensure high sensitivity in the range of low flow rate. Furthermore, because the temperature of the heat generating resistor is so controlled as to remain constant regardless of change of the flow rate through a feedback control, nonnegligible error is incurred in the flow rate detection due to lag in response of the temperature detecting resistor notwithstanding of high responsivity of the heating current, thus giving rise to a problem.
On the other hand, the flow sensor of the heating current detection type generally exhibits preferred sensitivity to the change of the flow rate. However, in the range of low flow rate, the sensitivity of this type flow sensor is low because of nonnegligible heat losses due to heat conduction to the diaphragm supporting portion and the cavity portion when compared with the quantity of heat transferred to the flow of the fluid medium from the heat generating resistors, thus rendering it difficult to detect the fluid flow behavior over a wide range with reasonable accuracy. Certainly, the sensitivity of the heating current detection type flow sensor can be enhanced by decreasing the thickness of the diaphragm. In that case, however, the mechanical strength of the diaphragm will become enfeebled, giving rise to another problem. In other words, with regard to the size of the diaphragm, the flow sensitivity and the mechanical strength are, so to say, in a trade-off relation.
In the light of the state of the art described above, it is an object of the present invention to provide a flow rate detecting element of the heating current detection type which is designed optimally by taking into consideration both factors of the mechanical strength and the sensitivity.
In view of the above and other objects which will become apparent as the description proceeds, there is provided according to a general aspect of the present invention a heat generation type flow sensor which includes a silicon substrate, a diaphragm disposed on the silicon substrate and having a cavity formed in a surface thereof, a flow rate detecting element provided on the diaphragm and including a heat generating resistor for outputting an electric signal indicative of a heating current flowing through the heat generating resistor, a supporting member for supporting the flow rate detecting element on the diaphragm in such a disposition that one surface of the diaphragm is exposed to a fluid for measurement while the fluid for measurement is difficult to flow into the cavity formed in the other surface of the diaphragm, and a control unit for performing such control that temperature of the heat generating resistor is sustained higher by a predetermined temperature than that of the fluid for measurement, wherein the heat generating resistor and the diaphragm are so dimensioned that ratio of a width of the heat generating resistor to that of the diaphragm is in a range from 0.4 to 0.6 inclusive and that ratio of a length in a longitudinal direction of the heat generating resistor to that of the diaphragm is in a range from 0.4 to 0.6 inclusive.
By virtue of the structure of the heat generation type flow sensor described above, the flow-rate sensitivity thereof can be enhanced without increasing the size of the diaphragm.
In a preferred mode for carrying out the present invention, the length in the longitudinal direction of the diaphragm may be so dimensioned as to be at least double the width of the same.
With the arrangement mentioned above, the flow sensitivity can be enhanced while ensuring a sufficient mechanical strength for the diaphragm.
The above and other objects, features and attendant advantages of the present invention will more easily be understood by reading the following description of the preferred embodiments thereof taken, only by way of example, in conjunction with the accompanying drawings.